The invention relates to an optoelectronic integrated semiconductor device including a separator of the TE and TM polarizations, this separator comprising two parallel and monomode optical guides, one of which receives at the input a luminous signal, comprising means for carrying out the separation of this signal in its two TE and TM components, one of which (TE) is transported to the output through one of the guides and the other (TM) is transported to the output through the other guide.
The invention is used inter alia in the manufacture of devices intended for use in homodyne or heterodyne detection.
Especially, the homodyne detection has for its object to mix a signal with a signal of a local oscillator of given frequency to extract therefrom the intermediate frequency. For this purpose, it is indispensable to have available both the TE (Transverse Electric) component and the TM (Transverse Magnetic) component of the signal. In fact, in the homodyne detection, the intensity of the resulting field should be measured, which is expressed by the square value of the sum of the field of the signal and of the field of the local oscillator. In the expression of the intensity of the resulting field the scalar product of the field of the signal and of the field of the local oscillator is employed. It can be seen that, when the vector of the electric field of the signal is orthogonal to the vector of the electric field of the local oscillator, no detection can take place because the information is contained in the scalar product. This explains that it is necessary to decompose both the signal to be treated and the signal of the local oscillator into their two components TE and TM because in these conditions always the case arises in which at least one of the two scalar products then obtained is not zero.
A device for separating the polarizations TE and TM is known from the prior art by the publication entitled "LiNbO.sub.3 coupled wave guided TE/TM Mode Splitter" by Osamu Mikami in "Applied Phys. Lett. 36 (7), 1 Apr. 1980, 1980 American Institute of Physics, pp. 491-492.
This device comprises two parallel guides completely buried in a substrate of lithium niobate and formed by local diffusion of titanium (Ti) ions.
At the surface of one of the guides and over a given length L.sub.1, designated as coupling length, a metallic layer is formed directly on the said guide. At the surface of the other guide, a second metallic layer is formed over the same length, but in this case a dielectric layer is interposed between this metallic layer and the upper surface of the guide. A potential difference may be applied between the two metallic layers, which then form electrodes.
The operation of this device is based on the following effects:
It is known that, when two guides are arranged parallel over a given length, if one of the guides transports a given mode, this mode will completely pass into the second guide provided that the length over which the two guides are parallel corresponds to the coupling length for the said mode.
This coupling length is associated with the inverse of the difference of the constants of propagation of the symmetrical and antisymmetrical modes, which can propagate in this structure, then forming a coupler.
Now, for each of the polarizations TE and TM, the constants of propagation of the symmetrical and antisymmetrical modes are practically identical, respectively. This results in that, if the TE and TM polarizations should be separated by a coupler constituted by identical guides, this would necessitate an almost inifinite coupling length.
Therefore, the aforementioned document discloses a separator of the TE and TM polarizations based on an asymmetrical structure, in which the coupler is symmetrical for the TE polarization and is asymmetrical for the TM polarization from the viewpoint of the constants of propagation of the TE and TM polarizations for each individual guide. This formation is based on the properties of the TM polarization, which differ from those of the TE polarization. It has been found that, when a metallic layer is directly arranged at the surface of an optical guide, the constant of propagation of the TM polarization is strongly modified by the presence of this metallic layer, while the constant of propagation of the TE polarization is slightly influenced by this presence.
On the other hand, it is known that, in order that two monomode guides are coupled, it is necessary that the constants of propagation of the fundamental mode in each guide are practically identical in the input guide or first guide and in the output guide or second guide, into which the said mode passes by coupling.
When these constants are identical or practically identical for the given mode, there is a total or substantially total transfer in the two guides of this mode from one guide into the other guide. The coupler is considered as symmetrical for the said mode.
When these constants on the contrary are different, the transfer is only partical, even zero, and the coupler is considered as asymmetrical for this mode.
It results from the respective properties of the TE and TM polarizations mentioned above that due to the presence of a metallic layer on the output guide or second guide, the constant of propagation in the guide provided with the metallic layer will become greatly different from the constant of propagation in the first guide not provided with the metallic layer as far as the TM polarization is concerned. On the other hand, the constants of propagation in one and the other guide will remain substantially identical as far as the TE polarization is concerned.
In this case, transfer of the TM polarization from the input guide or first guide to the output guide or second guide will therefore not take place, while the TM polarization will remain in the input guide, in which it will continue to propagate; this transfer can take place for the TE polarization, which will therefore pass into the output guide or second guide or the two parallel guides.
The differences in properties between the TE polarization and the TM polarization in lithium niobate are due inter alia to a physical phenomenon, which appears more particularly in this material. The said phenomenon consists in the appearance of a plasma mode of the TM type, which has an effective propagation index considerably higher than all the remaining indices of the structure and which renders the coupling asymmetrical when the metallic layer is present on the input guide.
In the known device, the second metallic layer is disposed on a dielectric layer in order not to influence the said phenomenon. The metallic layers can thus serve as electrodes in order to adjust the coupling length of the TE polarization to a length desired for the device.
However, if the device described in the aforementioned document has advantages, especially due to the fact that the input and output guides of the TE/TM separator are parallel, on the contrary it has the disadvantage of being formed from lithium niobate, which is not a material permitting of forming optoelectronic circuits in manufacturing synergy with the integrated circuits generally joined thereto.